Porous thin film materials, including polymers, ceramics, and composites, are of increasing technological and commercial importance. The full potential of such materials can be realized only if they are well characterized, and such characterization should include accurate measurement of total surface area and pore size distribution. Adsorption isotherms, in which the extent of adsorption is measured as a function of adsorbate partial pressure, are now widely used to characterize bulk porous samples. Nitrogen gas at its boiling point is the most commonly used adsorbate because it gives more consistent results than other adsorbates for a wide variety of adsorbent materials. Argon gas at its boiling point is a second commonly used adsorbate that gives reliable results.
Several commercial instruments are available for obtaining N.sub.2 adsorption isotherms. These instruments measure the amount of adsorbed N.sub.2 using gravimetric, volumetric, or dynamic flow-through methods. To determine sample surface area, the experimental isotherm can be compared to the BET model, developed by Brunauer, Emmett, and Teller, (J. Am. Chem. Soc. 1938, 60, 309) which models multilayer adsorption using one binding energy between the adsorbate and the surface for the first monolayer, and a second binding energy for adsorption of subsequent monolayers. The resulting isotherm can be given by: ##EQU1## in which n is the number density of adsorbed molecules, n.sub.m is the density corresponding to one monolayer on the available surfaces, p is adsorbate partial pressure, p.sub.o is adsorbate saturation pressure, and c is a constant that depends on the two binding energies. Since the amount of adsorption often depends on whether p is increasing or decreasing, adsorption is typically monitored as p/p.sub.o increases from zero to a value near one and then returns to zero.
Equation 1 can be arranged to give: ##EQU2##
When the BET model holds, a plot of .beta. vs. p/p.sub.o is a straight line whose slope (s) and intercept (I) can be used to evaluate c=1+s/I and n.sub.m =1/(s+I).
The molecular area of an adsorbed N.sub.2 molecule is well-known (a.sub.m =16.2 Angstrom.sup.2) and is generally independent of adsorbent properties. Sample surface area (A) is calculated using A=n.sub.m a.sub.m. Pore size distributions can also be obtained by analyzing the isotherm to determine the volume of capillary condensation occurring as a function of p/p.sub.o. (S. Gregg et al., Adsorption, Surface Area and Porosity, Academic Press, 1982, p. 132.)
Because current commercial technology for obtaining N.sub.2 isotherms requires a minimum sample surface area of 10.sup.4 cm.sup.2, it is most readily applied to high surface area bulk samples which often have several hundred square meters of surface area per gram. To make surface area measurements directly of thin films, where the surface area may be only an order of magnitude greater than the nominal film area, large areas (&gt;1000 cm.sup.2) of film are required. These areas can sometimes be obtained by depositing the film on a high surface area substrate. However, besides requiring additional preparation time, such samples may have surface areas which differ from that of a thin film formed on a planar substrate; the typical situation for most thin film applications. Enhancement of the sensitivity to adsorbed N.sub.2 by several orders of magnitude would allow full characterization of the surface area and pore size distribution of as-deposited thin films.
Recently, the extreme sensitivity of surface acoustic wave (SAW) devices to small changes in adsorbed mass has been utilized to construct a variety of chemical sensors. Measurement of as little as 100 pg/cm.sup.2, corresponding to 0.35% of a monolayer of N.sub.2 on the flat SAW device substrate, has been demonstrated. A related device, the quartz crystal microbalance (QCM), has been used to monitor adsorption of various species onto metal substrates. In comparison to the QCM, SAW devices provide enhanced sensitivity as a result of higher operating frequencies and confinement of the wave energy to within one wavelength of the surface. An additional advantage for the study of some thin film materials is that films are deposited on the oxide surface (e.g. SiO.sub.2, LiNbO.sub.3, or ZnO) of the SAW substrate rather than on the metal electrode of the QCM.
S. Martin et al., "Isothermal Measurements and Thermal Desorption of Organic Vapors Using SAW devices", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-34, No. 2, March 1987, pp. 142-147, describes earlier work where a SAW device was used to measure adsorption isotherms of organic materials carried on a nitrogen stream to the substrate surface of the SAW device.
S. Martin et al., "Acoustic Wave Devices for Sensing in Liquids", Transducers '87, June 1987, discloses other work where a bulk acoustic wave, not a surface acoustic wave, was used to sense mass changes caused by an electroplating process on a thin film electrode on the surface of the acoustic device. This earlier work is also shown in U.S. patent application Ser. No. 187,776, filed Apr. 29, 1988.